Accelerometer method and apparatus for integral display and control functions

ABSTRACT

Method and apparatus for detecting mechanical vibrations and outputting a signal in response thereto. An accelerometer package having integral display and control functions is suitable for mounting upon the machinery to be monitored. Display circuitry provides signals to a bar graph display which may be used to monitor machine conditions over a period of time. Control switches may be set which correspond to elements in the bar graph to provide an alert if vibration signals increase in amplitude over a selected trip point. The circuitry is shock mounted within the accelerometer housing. The method provides for outputting a broadband analog accelerometer signal, integrating this signal to produce a velocity signal, integrating and calibrating the velocity signal before application to a display driver, and selecting a trip point at which a digitally compatible output signal is generated.

This application is a division of application Ser. No. 07/901,626, filedJun. 19, 1992, now U.S. Pat. No. 5,517,183.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

TECHNICAL FIELD

The present invention relates to apparatus and method for monitoringmachine vibration. More particularly, the present invention is directedto an accelerometer package containing a vibration transducer with anintegral and improved display and control function.

BACKGROUND ART

Vibration analysis has been used for years to provide a determination ofthe proper functioning of different types of machinery, includingrotating machinery and rocket engines. A determination of a malfunction,if detected at a relatively early stage in its development, will allowchanges in operating mode or a sequenced shut down of the machineryprior to a total failure. Such preventative measures result in lessextensive and/or less expensive repairs, and can also prevent asometimes catastrophic failure of equipment.

Standard vibration analyzers are generally rather complex and difficultto transport, on a routine basis, to machinery for vibration analysispurposes. Due to this logistics problem and the necessary complexity ofsuch systems, a high cost is often associated with standard vibrationanalyzers. If vibration analyzer systems are permanently positionedadjacent machinery to be monitored, they may be used to provide controlsignals in response to vibration detected. However, if the controlcircuitry is located some distance from the vibration analysis system,there may be errors produced due to ground shifts/ground loops whichtypically occur in the harsh and electrically noisy operatingenvironments in which rotating machinery operate. Ground loops/groundshifts, as used herein, are voltage differences between various circuitgrounds. These voltage differences between grounds are typicallyvariable, unknown, and tend to cause false signals. If the detectorcircuitry for the vibration analysis equipment and accelerometertransducer are located some distance apart and/or powered by differentpower supplies, those components may also be subject to ground looperrors.

Less complex and costly hand held vibration analyzers typically do notprovide adjustable trip point signals to actuate alarms or shutdownequipment in response to sensed vibrations. Since consistent long termmeasurement of vibration may be sensitive to precise accelerometersensor location and orientation, it may be difficult to monitor longterm machinery condition using hand held equipment that does not providefor permanent fixture of the accelerometer sensor. The circuitry of handheld vibration monitoring equipment is generally not suitable forpermanent mounting to machinery to be observed.

A panel mounted accelerometer display is disclosed in U.S. Pat. No.4,622,548 to J. R. Andres and D. D. Wilson. The G-force accelerationmeasuring instrument is designed for use in aircraft, spacecraft,submarine, or other vehicles, and employs electronic sensing and displaytransducers for increased reliability with relatively small size andsmall mass. The instrument includes a bar graph display, preferablyincorporating liquid crystal transducer elements. It uses a solid stateor limited motion acceleration transducer and provides graphic andnumeric indication of the present G-force value together with numberindication of historic maximum and minimum G-force values. Theinstrument also includes gain adjustment arrangements for accommodatingdifferent instrument panel mounting and different types of vehicles withdifferent G-force capability, and has a signal output tap for recordingor slave indicating or telemetry or feed-back use. Since this device isdesigned to be panel mounted, there are no provisions for retro-fittingit to an existing commercial accelerometer to form an integral unit tosave costs of construction. No means are disclosed for vibrationisolation of the electronics. For this reason it is probably notsuitable for direct "on machine" mounting. It is also rather bulky forsuch installation. While the unit is of small size relative to otherpanel mount devices, it is relatively large and bulky for machinemounting. The electronics may be too complex for "on machine" mountingand include analog to digital components as well as sample and holdcircuits. There is no means for providing a digital trip signal or forvarying the conditions which would produce such a trip signal. Novelocity signal output is available. While the scales may be changed,the device is limited to linear scales and cannot readily be changed formonitoring with logarithmic scales.

Purely mechanical accelerometers are available for monitoring purposesbut do not have a means for providing electrical error or trip signals.Purely mechanical accelerometers may be difficult to read unlessexternal lighting is provided. Mechanical accelerometers have a fixedscale so that different accelerometers must be used for differentmachines depending on the expected range of vibration to be measured.Typically, there is no convenient means to change the range ofmeasurement of a mechanical accelerometer.

U.S. Pat. No. 4,198,864 to D. S. Breed discloses a mechanical apparatusfor sensing and recording changes in the velocity of a vehicle involvedin a crash. It is comprised of a sensing mass biased to an initialposition. The biasing force is only overcome by an acceleration having acertain magnitude or greater. The mass will move a distance proportionalto the force and will remain in position if the magnitude and durationof force are sufficient to overcome preset values. This device issuitable for measuring forces in a single catastrophic occurrence but isnot suitable for on-going monitoring of equipment.

U.S. Pat. No. 4,470,302 to N. E. Cart discloses a mechanical shippingaccelerometer having a transparent tube within which an inertial massmoves indicators relative to a fixed scale affixed to the transparenttube to provide an indication of maximum shock incurred in eitherdirection of its longitudinal axis. Springs on either side of theinertial mass maintain the inertial mass in its initial position. Aswith other mechanical accelerometers, the absence of an electronic tripsignal, the ability to change scales, and a lack of an illuminated scalelimits its usefulness.

Consequently, a need exists for improvements in accelerometer electronicdisplays and control functions which are more suitable for operationdirectly on machines and which are not so expensive and complex. Thoseskilled in the art have long sought and will appreciate the novelfeatures of the present invention which solves these problems.

STATEMENT OF THE INVENTION

The present invention is directed, in a preferred embodiment, to animproved accelerometer package containing not only a transducer but alsodisplay electronics and a switch selectable trip level for providing afault signal. The resulting packaged system may be incorporated withinan accelerometer case during the original manufacturing process, or maybe retro-fitted into commercially available accelerometers. Micrologicand surface mount technology may be employed to significantly reduce thepackage size. Small mass and size are important to package constructionwhich is designed for machine mounting directly adjacent thosemechanical components whose vibration is to be monitored.

The present invention, in a preferred embodiment, includes a lineardisplay driver for driving an electronic bar graph display. The lineardisplay driver is pin for pin compatible with a logarithmic displaydriver so that a substitution may be made to provide a wide dynamicrange. With a 10 segment readout which provides 3 db/segment, a dynamicrange of 30 db is available. A dip switch component is also available sothat any segment may be selected and used to provide a digitallycompatible trip signal which is contemplated to be optically isolated soas to avoid the problem of ground loops which may be present. In apreferred embodiment, the accelerometer output is integrated to providea velocity signal and has a velocity signal output for use external tothe package. A broad band accelerometer output is also available forexternal use. A potentiometer is used for calibrating the device basedon accelerometer calibrations which are normally supplied by themanufacturer of the accelerometer. A housing with two windows is used inthe preferred embodiment so that the bar graph is directly visible fromthe first window and ready access is available from the second window tothe dip switches for the purpose of setting a trip level. In a preferredembodiment, two low durometer internal O-rings are used to providevibration damping for the retro-fit electronic package.

Accordingly, it is an object of the present invention to provide adirect display of "G" levels from the accelerometer package which may bemounted directly on the machine component to be monitored.

A feature of the present invention is the ability to select "G" leveltrip points for providing a digitally compatible error signal withoutthe need for complex processing equipment.

An advantage of the present invention is the low cost for a compactpackage that includes features that normally are provided only with morecomplex and expensive vibration monitoring equipment.

These and other objects, features, and advantages of the presentinvention will become apparent from the following detailed description,wherein reference is made to the figures in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an accelerometer display andcontrol in accord with the present invention;

FIG. 2 is an elevational view, partially in section, of an accelerometerdisplay and control in accord with the present invention; and

FIG. 3 is an elevational view, from the opposite side as shown in FIG.2, of an accelerometer display and control in accord with the presentinvention.

While the invention will be described in connection with the presentlypreferred embodiment, it will be understood that it is not intended tolimit the invention to this embodiment. On the contrary, it is intendedto cover all alternatives, modifications, and equivalents as may beincluded in the spirit of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to an accelerometer package thatincludes an integral display and control. A preferred embodimentaccelerometer package 10 may be seen in FIG. 2 and FIG. 3. FIG. 2 andFIG. 3 provide views of accelerometer package 10 from opposite sidesrotated 180° so that windows 12 and 14 may be observed. Within window 14can be seen electronic bar graph 16 which is composed of discrete bargraph elements 18. Elements 18 may include individual light emittingdiodes or liquid crystal elements or other such display elements as maybe suitable. In applications where the lighting is not conducive toclear viewing of displays, it may be more desirable to use lightemitting diodes or provide additional display lighting. As theaccelerometer signal amplitude increases as a result of increasedvibrations, a greater number of elements 18 on display 16 will beactivated. Since accelerometer package 10 is designed to be mounted tomechanical equipment to be observed, long term monitoring of mechanicalequipment is possible with some assurance that long term changes seen inbar graph 16 are caused by mechanical differences rather than themechanical coupling or exact placement and orientation of theaccelerometer sensor with respect to the monitored machinery.

In a preferred embodiment, accelerometer section 20 is a commercialaccelerometer to which a retrofit package is added. Accelerometerpackage 10 may be built as a single unit during manufacture. Micrologictechnology could be employed during manufacture so as to create anoverall smaller package. Circuit board 22 is shock-mounted in body 24with low durometer O-rings 26 which are seated in an internalcylindrical wall 27 of the body 24. Other techniques for mountinginclude washers, springs, rubber feet, etc.

Window 12 in FIG. 2 discloses dip switch package 28 composed of tenindividual switches 30. In this embodiment, individual switches 30 maybe selected to produce a digital trip signal at a chosen "G" forcelevel. As an example, if bar graph 16 is calibrated to read linearlyfrom 0 to 5 G's and it desired to have accelerometer package 10 producea digital trip signal for vibration of approximately 3.5 G's for controlpurposes, then switch 7 could be selected to achieve this response. Eachadditional switch represents an increment of 0.5 G's so that switch 7would be the appropriate switch for 3.5 G's. This operation is explainedin more detail subsequently in connection with the circuit diagram ofFIG. 1.

A typical application for this device could be local monitoring of amachine vibration level where a "G" level is selected at which someaction is desired, i.e. machine shut down, timing circuit activation,alarms, computer iteration, recording of signals, etc. When this featureis used, the digital signal is available at connector 32 through cable34. Various connectors may be used for this purpose and connector 32 mayinclude a lock screw 36 to secure connector 32 from backing off due tomachine vibration. Various means can be used to mount accelerometerpackage 10 to the machinery to be monitored and body 24 may be ofvarious shapes as desired. Micrologic and surface mount technology maybe employed to significantly reduce the internal circuitry size as wellas the size of body 24. Small packaging is especially desirable formachine mounting since a small mass and size will reduce the forces towhich accelerometer package 10 is subjected.

FIG. 1 discloses a preferred embodiment circuit diagram. Accelerometercircuit 40 may include various types of accelerometer sensors includingstrain gauges and piezoelectric gauges. A built in amplifier (not shown)may be used with the accelerometer to increase output to a desiredlevel. Power for the accelerometer is available from pin 1 of connector42. Power for digital circuitry is supplied at pin 2 of connector 42.Capacitor 44, shown with a value of 10 micro Farads, is used as a D.C.filter to remove power supply voltage from the accelerometer signalbefore application to display and control circuitry.

Although specific values of components are provided for a, preferredembodiment description of the present invention, it is understood thatin many cases different values may be used. As well, it is understoodthat different circuitry may often be used to provide equivalentresults. A broad band accelerometer signal is connected to pin 6 ofconnector 42 to make this signal available for use as may bedesired--for instance with accelerometer signal recording equipment.Diode 46 rectifies the accelerometer signal before it reaches integratorcircuitry which includes resistor 48 and capacitor 50 shown as havingvalues of 100 kilo-ohms and 1 micro-farad respectively. The integratedaccelerometer signal, or velocity signal, is connected to pin 7 ofconnector 42 so that it will be available for analysis or displayelsewhere as desired. In addition to integrating the accelerometersignal to produce a velocity signal, integrator circuitry provides atime constant to prevent a transient signal from tripping the system.While the values of resistor 48 and capacitor 50 shown provide a 0.1second time constant, component values may be adjusted to provide a 1-2second time constant to greatly reduce the possibility of transients.The larger time constant will also result in a steadier display at bargraph display 16. Variable resistor 52 is used to scale the signal forcalibration purposes when it is applied to display driver 54. Forinstance, if the accelerometer calibration data provides that five G'sequals five volts, then resistor 52 is adjusted so that a five voltinput will activate the number of bar graph elements 18 that aredesignated to indicate five G's. Since a full scale reading would beequal to five G's in this example, resistor 52 is adjusted so that alldisplay elements 18 are activated with a five volt input. The devicewill now read from zero to five G's in 0.5 G increments because thereare ten elements 18 in the preferred embodiment of the presentinvention. Resistor 56 is used to set display brightness so that whenbar graph display 16 of FIG. 3 uses light emitting diodes for elements18, resistor 56 may be used to set the brightness of these elements. Ifit is desired that the display have a variable brightness, then resistor56 may be a variable resistor.

Display driver 54 of the present embodiment has ten discrete outputswhich are labeled as levels 1-10. Each of these ten outputs is used todrive one bar graph display element 18. When bar graph display isreading full scale so that all ten bar graph display elements 18 areactivated, then all ten discrete display driver 54 outputs must also beactivated. As shown with the level one output of display driver 54, eachdiscrete display driver output 54 is connected through a resistor, suchas resistor 55 having a nominal value of 390 ohms, to a correspondinginput of bar graph display 16. As each discrete output from displaydriver 54 is activated, its corresponding output line is sinked toground. When level one output is activated and sinked to ground, thencurrent will flow through resistor 56 and allow current to flow throughthe first bar graph element 18 light emitting diode of bar graph display16. Each output from levels one through ten of display driver 54operates in a similar manner. Power to each bar graph element. 18 issupplied via connection 2 of connector 42. A discrete output fromdisplay driver 54 sinks its corresponding output line to ground toprovide a current flow path which results in activation of a bar graphdisplay element 18. Each discrete output of display driver 54 is alsoconnected to dip switch package 28 which was shown in FIG. 2 throughwindow 12. Display driver 54 level one discrete output line is connectedto the terminal at switch one, level two is connected to switch two,etc.

In operation, the level of bar graph display 16 can be observed prior toselecting a switch. Then a switch can be chosen which is above theindicated level for purposes of setting a trip point when theaccelerometer signal or vibration increases. As can be seen, a commonpole on each dip switch 1-10 of dip switch package 28 is connected topin 4 of connector 42. The signal from did switch package 28 can beoptically isolated as by optical isolator 58 to prevent ground loopsfrom interfering with the signal so as to cause false signals. Opticalisolator circuit 58 is intended to represent symbolically the additionof such a circuit and is not intended to show all connections or valuesof components which may be included in such a circuit such as a currentlimiting resistors, receiving circuits, etc. Optical isolator circuitmay be integral to accelerometer package 10 or may be located at thereceiving circuit. In operation, all switches in dip switch package 28would be left in the open position except the switch which is related tothe desired trip point level.

In summary, accelerometer 40 supplies a signal which is filtered bycapacitor 44 and is rectified before integration and scaling. Resistor52 is used to calibrate or scale the signal to accelerometer 40calibrations which are normally supplied by the manufacturer. Discreteoutputs from display driver 54 are directed to corresponding inputs ofbar graph display 16 and also dip switch package 28 to allow display andalso a trip level signal. If a logarithmic display is desired ratherthan a linear display, then a logarithmic display driver chip that ispin for pin compatible with display driver 54 may be directlysubstituted. In a preferred embodiment, display driver 54 is chosen tobe a LM3914 integrated circuit for linear display and is chosen to be aLM3915 integrated circuit for logarithmic display. Variable resistor 52can be used to calibrate the graph display when changing from linear tologarithmic display. The display can also be calibrated in decibels whenusing a logarithmic display driver 54 so that each display segment 18could have a value of 3 db. This scale provides a wide dynamic range ofoperation of 30 decibels.

The foregoing description of the invention has been directed to aparticular, preferred embodiment in accordance with the requirements ofthe patent statutes and for purposes of illustration. It will beapparent, however, to those skilled in the art that many modificationsand changes in the specifically described accelerometer package 10 withintegral display and control functions may be made without departingfrom the scope and spirit of the invention. Therefore, the invention isnot restricted to the preferred embodiment illustrated but covers allmodifications which may fall within the spirit of the invention.

We claim:
 1. Display apparatus responsive to a transducer for detectingmechanical vibration and outputting an analog signal in responsethereto, said display apparatus comprising:a transducer electricaloutput connection for receiving a transducer signal; a display driverfor generating digital signals and applying said digital signals to aplurality of output lines, said digital signals being responsive to theamplitude of said analog transducer signal; a bar graph displayconnected to said plurality of output lines, said bar graph displayhaving a plurality of optical display segments which are activatable inresponse to said digital signals; an output connector having a digitaltrip point signal contact; a plurality of switches for selectivelyinterconnecting said output connector and said display driver, at leastone of said switches having a first contact in common with said outputconnector digital trip point signal contact, said at least one of saidswitches having a second contract in common with a corresponding one ofsaid plurality of output lines of said display driver, said at least oneof said switches including means for selectively connecting said firstand second contacts in common with each other; and means for opticallyand electrically isolating said trip point control signal.
 2. Theapparatus of claim 1, further comprising:each of said plurality ofswitches having a first contact in common with said output connectordigital trip point contact, each of said plurality of switches having asecond contact in common with respective ones of said plurality ofdisplay driver output lines, each of said switches including means forselectively connecting each respective first and second contact incommon with each other.
 3. The apparatus of claim 1, furthercomprising:an integrating circuity electrically connected with saidtransducer output electrical connection, said integrating circuitryhaving an integrator output connected to said display driver.
 4. Theapparatus of claim 1, further comprising:an optical isolating circuit,said optical isolating circuit being in common with said outputconnector digital trip point contact.
 5. The apparatus of claim 1,further comprising a housing having a cavity therein defined by aninternal wall: said display driver, said bar graph display, and saidplurality of switches each being disposed within said cavity; andsaidhousing having first and second windows, said bar graph display beingdisposed adjacent said first window in a position so as to be viewablethrough said first window and said plurality of switches being disposedadjacent second window in a position so as to be viewable through saidsecond window.
 6. The apparatus of claim 5, further comprising:meansdisposed between said internal wall of said housing and said displaydriver for shock mounting said display driver.
 7. Apparatus responsiveto a transducer electrical analog output signal representative ofmechanical vibration for producing a bar graph display, said apparatuscomprising:means for filtering a D.C. electrical component from saidtransducer electrical signal to produce filtered electrical outputsignal; means for integrating said filtered electrical output signal toproduce an integrating signal; means for calibrating said integratedsignal to produce a calibrated signal; means for producing from saidcalibrated signal a number of discrete digitally compatible electricaloutput signals wherein the number of said digitally compatibleelectrical output signals is a positive correlation function of theamplitude of said calibrated signal and each said digitally compatibleelectrical output signal is generated at a different discrete thresholdlevel of amplitude of said calibrated signal; an output terminal; meansfor applying a selected one of said digitally compatible output signalsto said output terminal as an electrically isolated trip signal whensaid calibrated signal reaches the level of amplitude at which saidselected digitally compatible output signal is generated; a plurality ofbar graph elements, each having an electrical input terminal andactivatable by application of an electrical signal to its inputterminal; and means for electrically coupling each of said plurality ofdigitally compatible output signals to a different one of the inputterminals of said bar graph elements and activating those of said bargraph elements which are coupled to the digital output signals whichexceed said trip level to thereby display said digitally compatibleoutput signals as a function of the amplitude of said calibrated signal.8. An apparatus as set forth in claim 7 said transducer is anaccelerometer.
 9. Apparatus responsive to a transducer analog outputsignal representative of mechanical vibration for producing a bar graphdisplay, said apparatus comprising:means for calibrating said transducersignal and for producing from said calibrated signal a number ofdiscrete digitally compatible electrical output signals wherein thenumber of said digitally compatible electrical output signals is apositive correlation function of the amplitude of said calibrated signaland each said digitally compatible electrical output signal is generatedat a different discrete threshold level of amplitude of said calibratedsignal; an output terminal; means for applying a selected one of saiddigitally compatible output signals to said output terminal as a tripsignal when said calibrated signal reaches the level of amplitude atwhich said selected digitally compatible output signal is generated;means for electrically and optically isolating said trip signal topreclude ground loop interference therewith; a plurality of bar graphelements, each having an electrical input terminal and activatable byapplication of an electrical signal to its input terminal; and means forelectrically coupling each of said plurality of digitally compatibleoutput signals to a different one of the input terminals of said bargraph elements and activating those bar graph elements which are coupledto the digital output signals which exceed said trip level to therebydisplay said digitally compatible output signals as a function of theamplitude of said calibrated signal.